Method of operating mercury cathode cells



United States Patent 3,109,796 METHGD 0F GPERATING MERCURY CATHODE CELLSJoseph L. Wood, Timonium, Md., and Glen P. Henegar,

Saltville, Va., assignors to Olin Mathiesou Chemical Corporation, acorporation of Virginia No Drawing. Filed Aug. 25, 1961, Ser. No.133,790

4 Claims. (Cl. 204-125) This invention relates to mercury cells for theelectrolysis of aqueous solutions of alkali metal compounds,particularly the halides, such as sodium chloride and has for itsprimary object the provision of a method of operation of a plant made upof a large number of such cells, particularly at the time the plant isstarted up.

Mercury cell plants comprise a large number of mercury cells and aconsiderable number of accessory parts. This invention is concernedprimarily with a method of installing anodes and starting up a plantconsisting of a large number of mercury cells. However, the method isapplicable to a relatively small number of cells or even to a singlecell having a plurality of anodes.

This invention is concerned particularly with stationary mercury cellswhich, for a variety of reasons, are relatively long and narrow. Suchcells preferably include a conducting bottom with a covering layer ofmercury as the cathode, an anode or series of anodes suspended in anelectrolysis compartment into the aqueous solution over the layer ofmercury. The aqueous solution, for example, a brine of sodium chloride,potassium chloride or lithium chloride and the mercury are introduced atthe slightly elevated inlet end. These flow through the compartment tothe outlet end while undergoing electrolysis. Chlorine gas is releasedand an amalgam of the alkali metal is formed. The amalgam is laterdecomposed in another operation with the formation of hydrogen and, forexample, metal hydroxides or alcoholates. The mercury is returned to thecell to repeat the cycle.

The anodes of stationary mercury cells are usually "fabricated ofrectangular blocks of graphite to each of which is attached a lead-inwhich provides support for the suspension of the graphite in the aqueoussolutions above the mercury cathode. The lead-in also conducts thecurrent from outside the cell, for example, from a bus bar to thegraphite anode.

In such cells, high current densities are employed so that it iseconomical to place the anode close to the mercury cathode. However,graphite anodes tend to erode under the conditions of electrolysis. Thelower surfaces of the graphite anode recedes from the mercury cathode,increasing the inter-electrode distance and increasing the voltagerequired to span the gap through the aqueous brine; In order to maintainhigh electrical efficiency and low resistance in the cell, it istherefore desirable to adjust the elevation of the anode with respect tothe mercury cathode at frequent intervals.

Adjustment of anodes requires an amount of labor which is a materialcost in the operation of the cells. Since the rate of erosion of thegraphite anode-s varies materially, anodes require adjustment andreplacement at various times. A fixed labor force can maintain theanodes in adjustment during the operation since the large number ofanodes requiring attention averages out the labor requirement foradjustment and replacement.

A problem arises in starting up a new plant in which all of the anodesare new and as uniform as possible. No adjustment or replacement isrequired for a period of time but after certain elapsed time dependingon the graphite, the operating conditions in the cell and other factors,a large proportion of the anodes require adjustment at the same time.Moreover there will be sufficient uniformity in erosion that a majorproportion of the anodes will require replacement at the same time.These requirements interfere with the smooth operation of the plant andmay require the shutdown of a large number of cells at the same time.The operation of the plant is materially improved by the process of thepres ent invention which evens out the schedule for adjustment andreplacement of anodes and therefore the labor required is more uniformlyspread over a period of time. Furthermore the total operation is moreuniform because only a small number of cells are removed from service atany one time for the purpose of anode adjustment or replacement.

The method of the present invention comprises the original installationin a plurality of mercury cells of anodes of varying thickness. After aninitial period a considerable proportion of the anodes in the plantrequire adjustment but replace-ment of anodes is required only for asmall proportion of the anodes at any one time. The required adjustmentscan be made on a few anodes at a time even though some are less in needof adjustment than others. The major improvement of the process of thepresent invention is the distribution over a considerable period of timeof the greater task of replacing anodes.

In mercury cells the thickness of the anodes initially installed dependson the design of the cell. Ordinarily cells are built with sidewallscapable of accommodating graphite anodes varying in thickness from about3 to 8 inches or more. It is theoretically most advantageous to installoriginal anodes, no two of which have the same thickness. in this waythe maximum leveling of the requirement for replacement is attained.Since this is not feasible in commercial operation, the preferredoperation is more practical. The method of the present invention thuscomprises the method of producing chlorine and alkali metal amalgam inthe electrolytic compartments of mercury cathode cells which comprisessuspending in said electrolytic compartments initial graphite anodeshaving diiferent thicknesses between about 2 and about 8 inches in saidelectrolytic compartments, supplying aqueous alkali metal brine andmercury to said compartments and electrolyzing the brine to formchlorine gas, depleted brine and alkali metal amalgam. The said initialgraphite anodes are subsequently replaced, when worn by. electrolysis,by graphite anodes of uniform thickness, preferably anodes of themaximum thickness the design of the cell permits.

For example, in a cell built to accommodate anodes 6 inches thick, it ispreferred to install one third of the anodes 3 inches thick, /3 of theanodes 4 inches thick and /3 of the anodes 6 inches thick. The relativeproportions of anodes of each thickness may be varied and the specificthicknesses may be .varied to utilize graphite blocks commerciallyavailable or commercially feasible for this purpose.

It is a particular advantage of the process of the present inventionthat, in addition to the leveling out of the replacement schedule,unexpected improvements are effected in the operation of the plantparticularly with respect to reduced proportions of by-product hydrogenin the chlorine. Deleterious impurities in the brine were reduced andlower hydrogen content in the chlorine was obtained. The graphite fromwhich the anodes are made contained various minor impuritiesparticularly iron, vanadium, and other metals usually present in partsper million. In operation these metals are leached from the graphiterather rapidly. After a period of use the con tribution of these minormetallic impurities from the graphite to the brine is greatly reduced.Initially when all the anodes are new and of equal thickness, the brineis substantially contaminated by these metals. When as replacements.

ite is materially reduced and as a result the hydrogen in the chlorineis maintained at a low level even in the initial stages of operation. Asnew anodes of uniform thickness are introduced as repl cement anodes,the varying thickness of the remaining anodes and their varying age inthe cell results in a lower level of metallic contamination in thebrine, a chlorine product containing less by-product hydrogen andrequiring less of treating agents to remove the contamination in thepurification and resaturation of the brine for recycle. Sudden increasesin hydrogen content of the chlorine and the chance of explosion due tohydrogen in the chlorine is substantially reduced. In addition, thevoltage requirement of each cell is substantially the same as thevoltage requirement of every other cell. The resulting power consumptionof the plant is thus maintained essentially constant over long periodsof time. Overall plant operation efiiciency and safety are thusimproved.

Example I In a mercury cell plant comprising 100 stationary cells, eachcell requiring 20 anodes, a total of 2,000 anodes are required. *Inthese cells, 600 anodes 3 inches thick, 600 anodes 4 inches thick and700 anodes 6 inches thick were installed. Anodes of differentthicknesses were adjacent to each other so that each cell containedseveral anodes of each thickness. In the operation of the plant theanodes were adjusted as required from time to time and the anodes wereremoved and replaced when their thickness averaged about 1 inch. Thusthe original 3 inch thick anodes required replacement at the earliestdate, the 4 inch thick anodes at an intermediate date and substantiallyall of the 3- and 4-inch anodes were replaced before it became necessaryto replace any of the 6 inch anodes. Anodes, 6 inches thick were usedAfter the program of replacement began the requirements were greater atcertain times but by the'time all the anodes had been replaced, therequirements were uniformly spaced with respect to time.

Example II In a mercury cell plant of the same number of cells andanodes, as described in Example I, the original installation comprisedalternating anodes of 3 inches and 4 In a mercury cell plant comprising50 stationary cells,

4 each cell required 20 anodes, a total of 1,000 anodes being required.In one-third of these cells, anodes 3 inches thick were installed. Inanother one-third of the cells, anodes 4 inches thick were installed andin the remaining one-third, anodes 6 inches thick were installed. Ineach cell the anodes were rotated from end to end of the cell atintervals for more uniform wear. The anodes were removed and replacedwhen their thickness averaged about 1 inch. The original 3 inch thickanodes were replaced at the earliest date, the 4 inch thick anodes at anintermediate date and substantailly all of the 3- and 4-inch anodes werereplaced before it became necessary to replace any of the 6-inch anodes.Anodes, 6 inch thick were used as replacements. As each cell was shutdown, all the anodes in the cell were replaced. After the program ofreplacement began the requirements were greater at certain times but bythe time all the anodes had been replaced, the requirements wereuniformly spaced with respect to time and the power requirement for theentire plant was substantially uniform.

What is claimed is:

1. The method of producing chlorine and alkali metal amalgam in theelectrolytic compartments of stationary mercury cathode cells whichcomprises suspending in said electrolytic compartments initial graphiteanodes having different thicknesses between about 2 and about 8 inches,at least one-quarter of 'said initial anodes having substantially thesame thickness, supplying aqueous alkali metal brine and mercury to saidcompartments and electrolyzing the brine to form chlorine gas, depletedbrine and. alkali metal amalgam, subsequently replacing said initialgraphite anodes by replacement graphite anodes, all of said replacementgraphite anodes having substantially the same thickness.

2. The method of claim 1 in which the said initial graphite anodes aresubsequently replaced by graphite anodes having the maximum thicknessthe design of the cell permits.

3. The method of claim 1 in which at least one-quarter but not more thanthree-quarters of the anodes initially installed have substantially thesame thickness and the balance of the anodes have a substantiallydifferent thickness.

4. The method of claim 1 in which all the anodes in any one of the saidmercury cathode cells have the same thickness and at least one-quarterbut not more than three-quarters of the said cells contain anodes of thesame thickness.

References Cited in the file of this patent UNITED STATES PATENTS2,704,743 Deprez Mar. 22, 1955 FOREIGN PATENTS 708,023 Great BritainApr. 28, 1954 402,319 Italy Feb. 27, 1943

1. THE METHOD OF PRODUCING CHLORINE AND ALKALI METAL AMALGAM IN THE ELECTROLYTIC COMPARTMENTS OF STATIONARY MERCURY CATHODE CELLS WHICH COMPRISES SUSPENDING IN SAID ELECTROLYTIC COMPARTMENTS INITIAL GRAPHITE ANODES HAVING DIFFERENT THICKNESSES BETWEEN ABOUT 2 AND ABOUT 8 INCHES, AT LEAST ONE-QUARTER OF SAID INITIAL ANODES HAVING SUBSTANTIALLY THE SAME THICKNESS, SUPPLYING AQUEOUS ALKALI METAL BRINE AND MERCURY TO SAID COMPARTMENTS AND ELECTROLYZING THE BRINE TO FORM CHLORINE GAS, DEPLETED BRINE AND ALKALI METAL AMALGAM, SUBSEQUENTLY REPLACING SAID INITIAL GRAPHITE ANODES BY REPLACEMENT GRAPHITE ANODES, ALL OF SAID REPLACEMENT GRAPHITE ANODES HAVING SUBSTANTIALLY THE SAME THICKNESS. 